Remote surveillance and fault location unit for pulse regenerator repeaters

Abstract

A remote surveillance and fault location unit for installation in each pulse generator repeater in a digital transmission link uses a service signal added to the digital data signal conveyed by the link. Each unit comprises a receiver circuit (100), a time base (200) and a transmitter circuit (300) suitable for implementation as an integrated circuit. The units add data representative of the performance of the repeater at which they are located to the service signal which is then monitored at one end of the link. The remote surveillance and fault location unit comprises a receiver circuit connected to an input to the pulse generator repeater for receiving a service signal if one is present at the input, a carrier frequency and clock bit frequency recovery circuits, a transmission circuit having its output connected to a second input of the pulse regenerator repeater for transmitting a service signal corresponding to the received frame to which data has been added.

We claim: 
     
       1. A remote surveillance and fault location unit for implementing the method of remote surveillance and fault location of pulse regenerator repeaters in a digital transmission link using a service signal added to the digital data signal conveyed by the link, the service signal being relayed at each pulse regenerator repeater and being constituted by a low frequency carrier modulated by a very low frequency binary signal in the form of a repetitive frame comprising a synchronizing-word followed by time slots in which there are written in order, on each occasion that the service signal is relayed, data relative to the error rates of the pulse regenerator repeaters, the said unit being connected, in the link, in parallel with a pulse regenerator repeater which is provided with an error rate measuring circuit and service signal separator units arranged on either side of the pulse regenerator repeater in question for one transmission direction the said remote surveillance and fault location unit comprising: a receiver circuit having its input connected via one of the separator units to a first input to the pulse regenerator repeater for receiving a service signal should one be present at this input, the said receiver circuit comprising a pulse-shaping circuit followed by an envelope detector circuit for detecting the frame by demodulating the received service signal, and a first shift register having a serial input and both serial and parallel outputs to make it possible to access a plurality of digits in the frame in parallel, and a synchronizing word decoder connected to the parallel output of the first shift register,   a time base comprising a carrier frequency recovery circuit constituted by an oscillator synchronized by the output signal from the pulse-shaping circuit, and a clock bit recovery circuit for recovering the bit frequency of the received frame, and being used to control shifting of the first shift register, and   a transmission circuit having its output connected via a second separator unit to a second input to the pulse regenerator repeater for transmitting a service signal corresponding to the received frame to which data from the error rate measuring circuit has been added, the said transmission circuit comprising a second shift register having both serial and parallel inputs and serial and parallel outputs, and connected by its serial input to the serial output of the first shift register via a logic gate, having its parallel input connected to the outputs of the error rate measuring circuit, having its parallel output connected to the outputs connected to the input of a free-time-slot decoder and having its serial output connected to a modulator, the said logic gate being opened by the synchronizing word decoder on detecting sychronizing word and closed by the free-time-slot decoder on detection of a free channel, the said free-time-slot decoder also controlling an enable-input to enable parallel loading of the second shift register on detection of a free channel, and the said modulator receiving the signal to be modulated from the carrier frequency recovery circuit and the modulation signal from the serial output of the second shift register.   
     
     
       2. A unit according to claim 1, further comprising: a local synchronizing-word memory,   a multiplexer selectively connecting the parallel input of the second shift register either to the output of the local synchronizing-word memory or to the output of the error rate measuring circuit,   a synchronizing-word-absence detector connected to the output of the synchronizing-word decoder, and   a frame clock which provides a frequency T at the frame repetition rate, which, under the control of the synchronizing-word-absence detector and when such an absence is detected, replaces the synchronizing word decoder in the control of the logic gate that connects the serial output of the first shift register to the serial input to the second shift register and which controls the multiplexer in such a manner as to periodically connect the output of local synchronizing-word memory to the parallel input of the second shift register.   
     
     
       3. A unit according to claim 2, further comprising: a second multiplexer inserted between the output of the error rate measuring circuit and the input to the first-recited multiplexer and addressed by means of a third multiplexer, either by the synchronizing-word decoder or, in the absence of a detected synchronizing-word, by an address generator controlled by the frame clock,   the said third multiplexer, and   the said address generator.